Internal Structures

This first article is intended to illustrate the internal structures of a Digital Single Lens Reflex camera. In my opinion, it is vital to get a general overview of the most important elements involved in order to understand how they contribute to the formation of an image. Also, knowing which task each component has to carry out may help to comprehend the more detailed descriptions on the optical and electronical elements later. The image below shows a cross-section through a Canon EOS 1D Mark IV, a very top-level DSLR camera.

Some of the components are described in the image, however the complexity of a DSLR camera requires some more explanations.

Probably the most unique component of a DSLR camera is the reflex mirror. The main purpose of the reflex mirror is to reflect the incident light towards the viewfinder optics in the upper part of the camera. Being deflected and corrected in rotation by a pentaprism, light will be redirected to the eyepiece so that the photographer gets a picture of the scene from the perspective of the camera lens itself. This is a very helpful feature because actually seeing through the main lens avoids the picture to change from parallax error and also takes filters into account, if applied to the lens.

By way of qualification, it should be noted however, that mirrorless cameras almost always provide a live-view-function that does not suffer from parallax either. the live-view function projects an image onto the built-in LCD screen for observation during image composition. As this image is caught by the camera sensor directly, no different angle is involved and the photographer can see the picture as close to the final result as possible. One disadvantage of an LCD screen is that it is likely to suffer from external influences such as reflections from bright surroundings, distorting the impression of the live-view image.

Nevertheless, the mirror construction of a DSLR camera has some more advantages. As aforementioned, the reflex mirror deflects indicent light towards the viewfinder optics. Shortly before entering the pentaprism, a condenser lens – also referred to as a beamsplitter lens – will split the entire picture and divert it to a slightly different direction. This second beam of light is not intended to run through the eyepiece but to illuminate a special metering sensor that takes a reading on the brightness of the scene. The metering sensor is responsible for the correct exposure of an image in case that a semi- or fully-automatic mode is active. The metering sensor will be described in the Shutter article.

One other significant advantage over mirrorless cameras is offered by another construction. The primary mirror is not fully reflective but has a semi-transparent area. Due to this partly transparency, light can reach a secondary mirror, placed behind the primary reflex mirror. This secondary mirror reflects light beams to the autofocus sensor unit, a complex arrangement of optical elements designed to tell the camera whether an image is in focus or not. The autofocus construction will be described in the Focus article. The image below summarizes the different paths of light while the mirror is in it’s idle position.

Once the shutter-release button is pressed, both the main reflex mirror and the secondary mirror flip up, exposing the rear area to light. The camera sensor is covered by the shutter that will quickly open after the mirror has reached the upper position. With the shutter opened, the sensor is finally exposed to the incident light, recording the scene. Some interesting facts on sensor physics and processing electronics can be found in the Sensor article. After the exposure, the shutter will close automatically, followed by the primary and secondary mirror folding down again. The image below shows the exposure of the sensor while the mirror is flipped up.

General signal processing

The drawing below is an overview of the processing units in a digital camera. Elements marked in green are optical and optoelectronical devices while the white boxes refer to electronic components such as microchips and circuit boards.